Hearing aids are worn by millions of hearing-impaired people. A hearing aid receives input sound, amplifies it, and retransmits the amplified sound to the ear of its user via a loudspeaker. The hearing aid uses a microphone to receive sound and a loudspeaker to retransmit it.
A microphone is a type of “displacement” sensor. The hearing aid represents only one of many important applications for displacement sensors; other applications include precision measurement systems, pressure sensors, and non-hearing aid microphones.
Advancements in microphone technology have led to vast improvements in the performance of state-of-the-art hearing aids. But there is still room for significant performance improvement.
Until recently, the microphones used in hearing aids have been “omni-directional” microphones. An omni-directional microphone typically comprises a single displacement sensor whose sensitivity is substantially uniform for sound arriving from any direction. As a result, hearing-aid wearers often have difficulty understanding speech in noisy environments due to the fact that hearing aids simply amplify all received sound, including background noise and echoes.
The use of a directional microphone offers a hearing aid user an improved ability to understand speech. A directional microphone has a sensitivity that is higher for sound that arrives from directions within a “reception cone” than outside of it. As a result, the use of a hearing aid with a directional microphone can: 1) improve the signal-to-noise ratio for desired speech by focusing reception in the direction of the person speaking; 2) reduce the effects of reverberation by attenuating sound arriving from directions outside the reception cone; and 3) reduce feedback effects that occur between the hearing aid's loudspeaker and microphone.
A typical directional microphone uses an array of omni-directional microphone elements. The operation of a microphone array as a directional microphone relies on the fact that sound propagates in the form of a pressure wave. Microphone directionality is derived by sensing a difference in amplitude of the received pressure wave (i.e., a gradient in pressure) between neighboring microphones in an array. When a sound wave arrives at the microphone array at an angle, each element in the microphone array senses the wave at a slightly different time. At one instant in time, therefore, each microphone element sees a different point on the wave. In other words, the wave hits each microphone element at a different “phase” of its wavelength.
Since the input sound hits each microphone in the array at a different phase, the output signal of each microphone element is also at a slightly different phase. A signal processor receives these output signals and applies an appropriate phase shift to each so that they combine constructively for sound received along only desired directions. In similar fashion, an appropriate phase shift can be applied to each output signal so that they combine destructively for sound received along all undesired directions. The microphone array, therefore, can have selectively-improved sensitivity for sound arriving from only certain directions (i.e., directionality). Through the use of a sophisticated signal processing system, this technique enables a directional microphone to: 1) discriminate input sound arriving from only one particular direction; or 2) follow a moving transmitter; or 3) scan the microphone system's surroundings to align to a particular direction.
The directional sensitivity of a microphone array is a function of the ratio between the spacing of the array elements and the wavelength of the sound being received. The larger this ratio, the better the directional discrimination that can be achieved by the directional microphone. Unfortunately, for many directional microphone applications in the prior-art, and in particular for hearing aid applications, the spacing between microphones in an array is limited to only a few millimeters, while the wavelengths of the sound waves in the range of human hearing are on the order of tens of millimeters. The phase difference between the responses of two neighboring microphones, therefore, is extremely small. As a result, the directional performance of prior art microphone arrays has been disappointing.
Other factors have served to degrade the performance of prior-art directional microphones as well. First, a large portion of the input signal is lost in the process of the determining the directional component. Thus, the signal-to-noise ratio of the overall output signal is diminished. Second, the small spacing between microphones results in a directional component that is much smaller than the omni-directional component of the output signal. Also, the directional component decreases as the frequency of the sound decreases. Third, the sensors used in prior-art microphone arrays act as independent microphones, each of which outputs a signal based on the sound that is incident upon it. Unfortunately, the individual microphones also behave as independent noise sources. As a result, the noise contribution of the microphones to the overall output signal increases as the number of microphones in the array increases. As a result of these factors, prior-art directional microphone systems exhibit a signal-to-noise ratio that is worse than that of each individual sensor in the system. Finally, any mismatch in an operational characteristic of the microphones in the array degrades the performance of displacement sensor array further.
A displacement sensor array that generates a directionally-sensitive output signal with at least some of: a high signal; a high signal-to-noise ratio; reduced cost; and reduced complexity, would, therefore, be a significant advance in the art.